So called "no-backs" are employed in mechanical drive systems where it is necessary to prevent an aiding load from over-running the drive or preventing an opposing load from reversing the drive. The environment in which no-backs perhaps are most frequently found is that of aircraft that employ electrical or hydraulic systems to power moveable aircraft control surfaces as, for example, slats or flaps. In this environment, no-backs are employed to prevent any movement of the control surface other than that which results from operation of a prime mover for an actuating mechanism for the control surface. A typical no-back has a releasable brake associated with the no-back output shaft as well as an input shaft connected to the prime mover and coupled to the output shaft. A coupling between the input and output shafts is employed and usually is a ball ramp mechanism which is operable, in response to the transmission of torque from the output shaft to the input shaft to engage the brake to prevent movement of the output shaft, and thus assure that the associated control surface will remain in the position in which it was originally placed by operation of the prime mover.
Desirably, activation/deactivation of the no-back will not require the use of hydraulic pressure because of the possibility of an aircraft hydraulic system failure as well as the need to be redundant. Consequently, electric power may be employed. Where, however, the electric power is employed to engage/disengage the no-back brake, power requirements will be high and the mass of the electrical actuator undesirably large.
It is also desirable that in some instances, torque can be transmitted from the output shaft to the input shaft. For example, in a typical control surface drive system there may be power drive units (PDU's) at each end of an elongated rotary drive train. The two PDU's are employed for redundancy purposes and a so called throughshaft will typically extend between the PDU's and operate a plurality of spaced actuators, all connected to the control surface. Typically, the two PDU's will be driven by two different hydraulic systems, also used for redundancy purposes. Should one system fail, it is necessary to be able to drive the entire throughshaft and associated actuators by the PDU whose hydraulic system remains operative. That, in turn, means that the throughshaft will be rotating from one end to the other, including at the output of the PDU whose hydraulic system had failed. If the no-back associated with that PDU cannot be disengaged, the throughshaft cannot be rotated by the PDU with the operative hydraulic system and the aerodynamic configuration of the aircraft control surface cannot be altered. One means to avoid this problem is to provide for cross connection of hydraulic systems so that both PDU's, while normally having their own hydraulic systems, can be operated by a common single hydraulic system in case one hydraulic system fails. This requires additional hydraulic conduit valving mechanisms and controls therefore which undesirably add to the weight and bulk of the aircraft.
The present invention is directed to overcoming one or more of the above problems.